Psoriasis is non infectious, inflammatory disease of the skin. It is characterised by well defined erythematous plaques with adherent silvery scales.  
1-3 % of the population has this disease. More common in Europe and North America.Often seen between 15 to 40 yrs.  
Aetiology  Real aetiology is unknown. Genetic: If one parent is affected the offspring has 25% chance to develop the disease. If both parents are affected the chances rise to 60%.The disease has been found to have a genetic basis. On HLA phenotyping of the Chandigarh patients, a positive association was seen with HLA-B17 . However, familial occurrence was detected in only 2%.  
Common Patterns of Psoriasis:  

Plaque pattern: It is the most common type. The individual lesions are well demarcated and are red with dry silvery white scaling. Most common sites involved are scalp, elbow, knee and lower back.

Guttate Psoriasis
: Usually seen in the children and adolescents. The individual lesions of the rash are small and scaly. The bouts of guttate psoriasis clear in months but the patient may develop plaque pattern later.

Geographical tongue seen in 3.8% and Blepharitis in 5%.  
Scalp: This site is often involved. Areas of scaling are inter-spread with normal scalp. Significant hair loss occurs only if there is gross involvement.
Nails: Involvment of nails is common. Thimble pitting, Onycholysis (separation of nail from nail bed), Subungal hyperkeratosis are main changes seen in the nails. Involvement of nails reflect the extent of the disease elsewhere.
Flexures: Involvement of flexures like axillary folds and submammary folds are not scaly but red, glistening and symmetrical.
Palms: Psoriasis in the palms is poorly demarcated and is poorly erythematous.
Less common types of psoriasis are Napkin psoriasis and Localised pustular psoriasis in the palms and soles consisting of numerous small sterile pustules on erythematous base.  
Complications: Psoriatic Arthropathy is seen in 5 %. There may be distal small joint involvement mainly terminal interphalangeal joints of hands and toes. Single large joint arthropathy may also be seen which mimmics rheumatoid arthritis. Sacroiliac joint and lumbar spine involvement may also be seen.  
Serum copper levels are found to be higher while serum zinc levels were lower in Psoriatics as compared to controls.Urinary tract cell counts were significantly higher in Psoriatics.  
 Psoriasis ~ Treatment